1. Field of the Invention
The invention relates generally to a hub drive with a hub mounted on a hub support and having an integrated brake device. The hub can be driven by a drive shaft with the interposition of a transmission.
2. Description of the Currently Available Technology
DE 42 28 746 A1 describes a generic hub drive in which the brake device consists of a spring-loaded multi-disc brake which, in the braking position, couples the faster-moving drive shaft to the slower-moving hub. The hub of this hub drive is non-rotationally connected to the web of the transmission, which is realized in the form of a planetary gear train. The sun wheel of the planetary gear train is non-rotationally connected to the drive shaft. The planetary gear train acts like a locking mechanism on account of the stationary ring gear, which is utilized in the braking position to generate a braking torque.
A braking device of this type can theoretically be used as a service brake and/or as a holding brake although, depending on the application, it has disadvantages. For example, if the brake is used as a service brake, i.e., to decelerate a vehicle that is moving at a determined speed, the generally very high drag torque of the slower-moving hub must be absorbed. Accordingly, on account of the small diameter of the brake discs, a large braking force must be applied in the axial direction, which is accomplished by a package of disc springs. To compensate for the braking force during normal operation of the vehicle, to release the brake a large and powerful release device is necessary, which in the hub drive of the known device is embodied in the form of a large cylinder-piston unit.
If the known braking device is used as a holding brake, for example to stop an excavator (mobile excavator) equipped with the generic hub drive when the work device (excavator bucket) is actuated, it is necessary, on account of the high biting forces of the excavator, to have a holding torque that far exceeds the braking torque that must be applied to stop the moving excavator.
This considerable holding torque is introduced into the hub support via the planetary gears of the gear train. While this generally does not represent any problem when the braking device is used as a service brake, when the braking device is used as a holding brake, on one hand the stresses on the transmission are increased, which reduces the useful life of the transmission, and on the other hand when the holding torque is being introduced into the transmission, there is a high transmission play on account of elastic deformations, which is reflected during work with the excavator in the form of unpleasant reactions to changes in the load. Consequently, a typical known hub drive is nothing more than a compromise that does not have satisfactory characteristics in either of the two operating situations described above.
In other known hub drives, only a service brake is located between the high-speed drive shaft and the input of the transmission. The drag torque that must be absorbed is thereby reduced by the upstream position of the transmission (or the braking torque generated by the service brake is amplified by the transmission). To also be able to build up sufficiently large biting forces, additional, external holding brakes are used. Such a system increases the size of the hub drive and results in increased time, effort and expense in terms of manufacture, assembly and maintenance.
Therefore, it is an object of this invention to provide a generic hub drive that has compact dimensions, the braking device of which has an improved function in the operating situations described above.